Harmonic generation from fundamental-wavelength radiation is extensively used for generating radiation having wavelengths in the UV region of the electromagnetic spectrum. By way of example, from fundamental radiation having a wavelength of 1064 nanometers (nm), second harmonic (2H) radiation having a wavelength of 532 nm (green) can be generated by frequency-doubling the fundamental-wavelength radiation in a first optically nonlinear crystal. The 2H-radiation can then be sum-frequency mixed the with unconverted fundamental-wavelength radiation in a second optically nonlinear crystal to provide third-harmonic (3H) radiation having a wavelength of about 355 nm, i.e., a wavelength in the UV region.
The 2H-radiation can also be frequency-doubled in an optically nonlinear crystal to provide fourth-harmonic (4H) radiation having a wavelength of 266 nm. More complex frequency conversion schemes using two sources of fundamental radiation and a combination of harmonic generation and sum-frequency-mixing steps to provide specific wavelengths less than 200 nm. The shortness of the ultimately generated wavelength is limited primarily by the UV-transparency of available optically nonlinear crystals.
Almost from the day such frequency-conversion schemes for UV generation were first deployed it was found that an exit surface of any crystal used to convert one or more visible or infrared wavelengths of radiation to UV radiation would progressively deteriorate in the area where the UV radiation exited the crystal. If UV radiation at one wavelength were converted to UV radiation at a shorter wavelength, both entrance and exit surfaces would deteriorate.
The deterioration has been attributed to condensation of solid contaminants on the crystal resulting from UV breakdown of gaseous contaminants in an atmosphere surrounding the crystal. This has led to proposals for locating crystal in an enclosure in which a vacuum is maintained or which is purged and backfilled with an inert gas. Such enclosures, however, typically cost significantly more than the crystals they are trying to protect, whatever the measure of success.
Many suppliers of frequency-converted UV lasers, since such lasers were first widely commercially available, have simply accepted the deterioration as inevitable. These suppliers provided various means such that when a crystal surface deteriorated in one spot to a point where laser performance was compromised, the crystal could be moved manually or automatically with respect to the beam of radiation being converted such that a fresh surface of the crystal was exposed to the beam. This is often referred to by practitioners of the art as “crystal-shifting”, and has provided an extended useful lifetime of the crystals, typically greater than two-thousand hours of operation.
In May of 2005, U.S. Pat. No. 6,890,474 entitled “Algorithm for Enhancing the Lifetime of Critical Components in a Laser System” was granted to Gruber et al. The critical components referred to are optically nonlinear crystals. This patent discloses dividing a surface of an optically nonlinear crystal into a matrix or grid (plural rows and plural columns) of contiguous rectangular areas which are described as “macro-spots” and have an area greater than the cross-section area of a beam being frequency-converted. The beam cross-section is described as a “micro-spot”. The crystal is moved with respect to the beam such that, within a macro-spot, the micro-spot is shifted around some arbitrary path in increments, which can be more or less than a beam-cross section, until the macro-spot has degraded to point where performance will be compromised. The crystal is then moved such that the beam falls in a different macro-spot and the micro-spot is shifted in that macro-spot until a deterioration condition is or will be reached.
It is taught that the selection of a next macro-spot after a previous one is used, can be according to some predetermined pattern such as an outward spiral, or can be arbitrarily selected. It is admitted, however, that certain macro-spots, i.e. grid or matrix elements, would be designated as not usable if such elements included crystal defects such as inclusions, striations, or some other artifact of growth, cutting or polishing a crystal. Perfect crystals are an exception rather than the rule. Nevertheless, a lifetime of 19,000 hours is claimed to have been reached for one experimental example.
It is not clear how the 19,000-hour lifetime achieved by Gruber et al. compares with lifetimes achieved by others using other crystal-shifting schemes. Comparison is difficult in any event, as lifetime is dependent on factors such as crystal material, UV-beam intensity, converted wavelength, and atmospheric-conditions, among others.
However, it is arguable that for any given set of circumstances, the method of Gruber et al. would have used less than all the useful area of a crystal surface at a time when a crystal would be designated as having reached its useful lifetime. A root-cause of this is the obligatory division of the crystal into the regular grid of rectangular macro-spots.
Laser beams do not normally have a rectangular cross-section. The cross-section is usually circular or slightly elliptical. Accordingly, it is not possible even with overlapping of a beam in a shifting pattern to cover all of a rectangular area while still maintaining the beam within the area as taught by Gruber et al. Some area will be left unused, for that reason alone. This could of course be mitigated somewhat by defining macro-spots that had a much larger area than a micro-spot. This, however, would exacerbate another area-wasting aspect of the regular grid, namely, avoiding crystal defects, which can not normally be expected to conveniently confine themselves to one particular grid-element or macro-spot.
One example of this is depicted in FIG. 1A. Here, an area of crystal surface is depicted as having a clear aperture divided into a regular grid of six rows (A-F) and six columns (1-6). It is assumed here that the clear aperture practically will not extend all of the way to the edges of the surface. An inclusion (defect) 10 is located on a common boundary of grid elements (macro-spots) B3 and C3. According to the scheme of Gruber et al. these two elements (cross-hatched in FIG. 1A) out of thirty-six would be made unavailable for scanning (shifting) a micro-spot because of a defect having an area less than the area of one element.
It is not unusual, however, to have more than one defect in a crystal. FIG. 1B depicts the grid of FIG. 1A but wherein an inclusion 12 causes elements B2, B3, C2, and C3 to made unavailable for use, and a scratch 14 causes elements D4, E4, and E5 to made unavailable for use. Here again the unavailable elements are cross-hatched. This means only about 80% of the area of the crystal would be used, less any percentage resulting from an inability to use all of the area of any otherwise-usable macro-spot because of a non-rectangular beam cross-section. There is a need for a crystal-shifting method that does not have the area-wasting deficiencies of the method of Gruber et al.